Heat Pump Distillation

ABSTRACT

A distillation column is disclosed. The column includes a plurality of rectification zones and corresponding stripping zones. Each rectification zone is linked to a heat pump or a stage of a heat pump. Overhead material from the top rectification zone is compressed and used to heat bottoms liquid from the bottom stripping zone. Similarly, overhead material from a lower rectification zone is compressed and used to heat liquid taken from the uppermost or top stripping zone. Optionally, overhead material from a middle rectification zone is compressed and used to heat liquid from a middle stripping zone. A single multiple stage heat pump compressor may be utilized as opposed to a plurality of heat pumps. Because the heat exchanger from each rectification-stripping zone pair has a lower duty, economical stab-in heat exchangers may be utilized.

This application is a Continuation of prior copending application Ser.No. 11/934,321 which was filed Nov. 2, 2007, the contents of which areincorporated herein by reference thereto.

BACKGROUND

1. Technical Field

A distillation process and apparatus is disclosed for separating two ormore components having different volatilities from a liquid mixturecontaining the components. The process and apparatus employs an improvedheat pump configuration including multiple heat pumps and multiplereboiler heat exchangers in combination with a folded column design ormodified dividing wall column design.

2. Description of the Related Art

Distillation is a method of separation that is based on a difference incomposition between a liquid mixture and a vapor formed from the liquidmixture. This difference in composition arises from the dissimilareffective vapor pressures, or volatilities, of the various components ofthe liquid mixture. Distillation normally involves partial or flashvaporization of the liquid composition followed by condensation of thevaporized material, usually in multiple vaporization/condensationsections known as stages.

The apparatus in which the separating occurs is usually a vertical,cylindrical vessel called a distillation column. Fractional distillationor fractionation involves the collection of separate fractions ofcondensed vapor, each fraction being segregated thereby permitting anoriginal liquid mixture to be separated into two or more productstreams. Rectification involves the enrichment of a lighter stream byintimate contact with counter-flowing liquid reflux within thefractionation column, typically above the feed inlet, which removesheavier materials from the lighter stream. Stripping, on the other hand,involves the enrichment of a heavy stream below the feed by intimatecontact with counter-flowing reboil vapors within the column, whichremoves lighter components from the heavier stream.

Distillation or fractionation columns normally contain internal devicesfor effecting vapor-liquid contact such as plates, trays or packings.Generally, the larger the number of plates or trays, the better theseparation. A theoretical plate or a “theoretical stage” in adistillation process is a hypothetical zone or stage in which twophases, such as the liquid and vapor phases of a mixture, establishequilibrium with each other. The performance of a distillation processdepends on having a series of equilibrium stages and is thereforeenhanced by increasing the number of theoretical stages (NTS).

The combining of two distillation columns into a single column having avertical partition, known as a diving wall column (DWC), provides forthe separation of the feed into a plurality of product streams. The useof DWCs can reduce the size or cost of the equipment needed to produceoverhead, bottoms, and sidedraw products. DWC designs and controlsystems for DWCs are disclosed in U.S. Pat. Nos. 6,558,515 and7,267,746. Despite the advantages of DWCs, their widespread use has notoccurred.

A problem associated with distillation processes in general is theemployment of relatively large pieces of equipment that are inefficientwith respect to energy consumption. While conventional distillationsystems employ a single reboiler at the bottom stage, it is known thatenergy efficiency could be increased by incorporating capital-intensivereboilers at multiple sections along the column. However, the capitalcost of adding multiple reboilers to conventional distillation columnshas been cost prohibitive.

Currently, with energy costs increasing, and with national andinternational pressures to reduce CO₂ emissions, there is a recentcompelling need for energy efficient distillation column configurations.

SUMMARY OF THE DISCLOSURE

In satisfaction of the above needs, an improved, energy-efficientdistillation apparatus is disclosed. The disclosed column whichcomprises a vessel having a top, a bottom and a continuous sidewallextending therebetween. The vessel is connected to a feed inlet. Thevessel comprises a plurality of separation zones including a toprectification zone, a lower rectification zone, a top stripping zone anda bottom stripping zone.

The top rectification zone is connected to a top overhead outlet and thetop overhead outlet is connected to a top overhead loop or circuit. Thetop overhead loop passes through a top heat pump and a bottoms heatexchanger before returning to the column. The bottom stripping zone isconnected to a bottoms outlet and the bottoms outlet is connected to abottoms loop or circuit. The bottoms loop passes through the bottomsheat exchanger where it is heated by the top overhead loop before itreturns to the column. Alternatively, the bottoms stripping zoneaccommodates the bottom heat exchanger in the form of a stab-in heatexchanger.

The lower rectification zone is connected to a lower overhead outlet andthe lower overhead outlet is connected to a lower overhead loop orcircuit. The lower overhead loop passes through a lower heat pump and atop heat exchanger before returning to the column. The top strippingzone is connected to a top stripping outlet which is connected to a topstripping loop or circuit. The top stripping loop passes through the topheat exchanger where it is heated by the lower overhead loop beforereturning to the column. Alternatively, the top stripping zoneaccommodates the top heat exchanger in the form of a stab-in heatexchanger.

In a refinement, the column is of a folded design wherein the toprectification zone is disposed laterally adjacent to the top strippingzone with a top dividing wall disposed therebetween and, the lowerrectification zone is disposed laterally adjacent to the bottomstripping zone with a lower dividing wall disposed therebetween.

In a refinement, the top overhead loop or top overhead zone is connectedto a top rectification zone product outlet.

In another refinement, the bottoms loop or bottoms stripping zone isconnected to a bottom stripping zone product outlet.

In another refinement, the lower overhead loop or lower rectificationzone is connected to a lower rectification zone product outlet.

In another refinement, the top stripping loop or top stripping zone isconnected to a top stripping zone product outlet.

In another refinement, the top heat pump and lower heat pump areseparate stages of a multiple stage heat pump.

In another refinement, the lower rectification zone is in fluidcommunication with the top stripping zone. In a further refinement ofthis process, liquid from the lower rectification zone is passed to thetop stripping zone and vapor from the top stripping zone is passed tothe lower rectification zone.

In another refinement, the disclosed distillation apparatus furtherincludes a middle rectification zone and a middle stripping zone. Themiddle rectification zone is connected to a middle overhead outlet,which is connected to a middle overhead loop or circuit. The middleoverhead loop passes through a middle heat pump and a middle heatexchanger before returning to the middle rectification zone. Similarly,the middle stripping zone is connected to a middle stripping outlet,which is connected to a middle stripping loop or circuit. The middlestripping loop passes through the middle heat exchanger where it isheated by the middle overhead loop before returning to the middlestripping zone. Alternatively, the middle stripping zone accommodatesthe middle heat exchanger in the form of a stab-in heat exchanger.

In a refinement, the middle rectification zone is disposed laterallyadjacent to the middle stripping zone with a middle dividing walldisposed therebetween.

In a refinement, the middle rectification loop or middle rectificationzone is connected to a middle rectification product outlet.

In another refinement, the middle stripping loop or middle strippingzone is connected to a middle stripping product outlet.

In another refinement, the top, middle and lower heat pumps are separatestages of a multiple stage heat pump.

In another refinement, the distillation apparatus includes a pluralityof middle rectification zones and a plurality of middle stripping zones.

In another refinement, each separation zone comprises a plurality oftheoretical stages.

In another refinement, the feed is connected to the lower rectificationzone.

An improved distillation method is also disclosed for at least partiallyseparating at least one lighter component from at least one heaviercomponent of a mixture comprising the lighter and heavier components.The disclosed method comprises feeding the mixture into a vesselcomprising a plurality of separation zones including a top rectificationzone, a lower rectification zone, a top stripping zone and a bottomstripping zone. The method further includes passing top overheadmaterial from the top rectification zone through a bottoms heatexchanger to heat the bottoms liquid and cool the top overhead material.The method further includes passing lower overhead material from thelower rectification zone through a top heat exchanger to heat the topstripping liquid and cool the lower overhead material.

In a refinement, the method further includes drawing off at least someof the top overhead material as product while a remainder of the topoverhead material is cooled and returned to the column.

In another refinement, the method further includes drawing off at leastsome of the bottoms liquid as product while a remainder of the bottomsliquid is heated by the overhead material passing through the bottomsheat exchanger.

In another refinement, the method further includes drawing off at leastsome of the lower overhead material as product while a remainder of thelower overhead material is cooled and returned to the column.

In another refinement, the method further includes drawing off at leastsome of the top stripping liquid as product while a remainder of the topstripping liquid is heated and returned to the column.

Other advantages and features will be apparent from the followingdetailed description when read in conjunction with the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosed method and apparatus,reference should be made to the embodiment illustrated in greater detailon the accompanying drawing, wherein:

The FIGURE is a schematic diagram of a folded column design with threerectification zones numbered 1-3 and three stripping zones number 4-6and the heat pumps and heat exchangers associated with eachrectification and stripping zone.

It should be understood that the drawing is not necessarily to scale andthat the disclosed embodiment is illustrated schematically. In certaininstances, details which are not necessary for an understanding of thedisclosed method and apparatus or which render other details difficultto perceive may have been omitted. It should be understood, of course,that this disclosure is not limited to the particular embodimentillustrated herein. For example, this disclosure is obviously notlimited to a column with three rectification zones and three strippingzones. A column with more or less than three of each type of zone fallswithin the scope of this disclosure.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

A “reversible” distillation process includes an infinite number oftheoretical stages and an infinite number of heat sinks disposed abovethe feed inlet as well as an infinite number of heat sources disposedbelow the feed inlet, which would result in monotonically decreasingvapor rate away from the feed tray which would result in a tapered orconical shape to the upper end of the column. The same would be true forthe lower end of the column. Of course, such a design would be costprohibitive. A reasonable approximation would include a conventionalcolumn design with several heat sources disposed above the feed that arematched with a number of corresponding heat sinks or exchangers disposedbelow the feed. Because not the entire vapor needs to be compressedacross the column, at least some energy savings would occur. However,such a construction, with the need to vary the column diameter, wouldresult in a capital outlay that would not be easily recovered withenergy savings.

To satisfy the need for a more energy efficient distillation apparatus,the apparatus 10 is disclosed. The apparatus includes a vessel structure11 equipped with a top 12, a bottom 13 and a side wall 14. In theexample shown in the FIGURE, three rectifications zones numbered 1-3 areshown next to three stripping zones numbered 4-6. The uppermost or toprectification zone 1 is disposed adjacent to the top or upper moststripping zone 4 with a dividing wall 15 disposed between the zone 1 andthe zone 4. The top rectification zone 1 is in communication with themiddle rectification zone 2 as shown by the phantom line between zones 1and 2. The middle rectification zone 2 is disposed laterally adjacent tothe middle stripping zone 5 with a dividing wall 16 disposed between thezone 2 and the zone 5. The middle rectification zone 2 is incommunication with the lower rectification zone 3 as shown by thephantom line between zones 2 and 3. The lower rectification zone 3 isdisposed laterally next to the bottom stripping zone 6 with a dividingwall 17 separating the zone 3 from the zone 6. The top stripping zone 4is in communication with the middle stripping zone 5 which is incommunication with the bottom stripping zone 6 as shown by the phantomlines between the stripping zones.

The dividing wall structures 15, 16, 17 prevent direct communicationbetween the rectification and stripping zones. Further, the dividingwall structures 15, 16, 17 are not intended to facilitate heat transferbetween the rectification zones 1-3 and the stripping zones 4-6.

Liquid material from the lower rectification zone 3 is passed throughthe line 19 and pump 20 to the top stripping zone 4 as shown in theFIGURE. Conversely, vapor from the upper stripping zone 4 is passedthrough the line 19 a to the lower rectification zone 3.

It will be noted that the vessel or column 11 need not be of the foldeddesign shown in the FIGURE. The column 11 may be upright, with aplurality of heat pumps and reboiler exchangers.

The feed line 18 injects the feed into the lower rectification zone 3.Alternatively, the feed can enter the upper stripping zone 4 asindicated by the line 18 a shown in phantom. Each rectification zone 1-3and each stripping zone 4-6 will include a plurality of theoreticallystages, the number of which will vary, depending upon the particularseparation being carried out. Further, in the embodiment shown, onlythree rectification zones 1-3 and three stripping zones 4-6 are shownalthough the number of rectification zones and the number of strippingzones may vary. Depending upon the particular separation, the number ofstripping zones may vary from two to several with a corresponding numberof stripping zones. Yet another possibility is to connect the feed tothe upper stripping zone 4.

The top rectification zone 1 is connected to a top overhead outlet 21that is connected to a top overhead line 22 that passes through a topheat pump 23 where the overhead vapors are compressed and pumped throughthe bottoms heat exchanger 24 before returning to the top rectificationzone 1 (or elsewhere) through the return line 25. As seen in the FIGURE,light product may be taken off of the line 25 to the product outlet 26.Hot pressurized vapor passing through the top overhead loop 22, 23, 25,with its increased condensation temperature, is used to heat bottomsliquid from the bottoms stripping zone 6. Specifically, the bottomsstripping zones 6 is connected to a bottoms outlet 27 that is connectedto a bottoms loop comprising the line 28 that passes through the bottomsheat exchanger 24 before returning the liquid through the line 29 to thebottoms stripping zone 6 (or elsewhere in the column) as shown. Thepressurized vapor from the rectification zone 1, with its elevatedcondensation temperature, drives the reboiler exchanger 24 resulting ina heating of the bottoms liquid flowing through the lines 28, 29. Abottoms product stream 31 may be taken off of the bottoms loop 28, 29.

Similarly, overhead vapor is taken from the middle rectification zone 2through the outlet 32 and passed through the middle overhead loop thatincludes the line 33 that passes through the middle heat pump 34 wherethe overhead vapor is pressurized before it is passed through the middleheat exchanger 35 and returned to the middle rectification zone (orelsewhere) through the line 36. The heated compressed vapors from therectification zone 2, with their increased condensation temperature, areused to heat liquid drawn off of the middle stripping zone 5.Specifically, the stripping zone 5 includes a bottoms outlet 37connected to a line 38 that passes through the middle heat exchanger 35before returning to the middle stripping zone (or elsewhere) through theline 39.

Finally, the lower rectification section includes an overhead outlet 41connected to a line 42 that passes through the lower heat pump 43 beforepassing through the top heat exchanger 44 and returning to the lowerrectification zone 43 (or elsewhere) through the return line 45. Thepressurized overhead vapor from the overhead rectification zone 3, withits increased condensation temperature, is used to heat bottoms liquidfrom the top stripping zone 4 which includes a bottoms outlet 47 that isconnected to a line 48 that passes through the top heat exchanger 44before returning to the top stripping zone 4 (or elsewhere) through thereturn line 51.

As shown in the FIGURE, three heat pump compressors 23, 34 and 43 areshown. However, the heat pump compressors 23, 34 and 43 need not beseparate pumps but can be an integrally geared multiple stage type wherea motor powers a plurality of wheels with each wheel having a separateinlet and outlet. These multiple stage compressors are known in the artand will not be described in detail here. Because vapor that originatesfrom an intermediate rectifying stage only needs a compression ratiothat provides a sufficient temperature driving force when it condensesto boil liquid from an intermediate stripping stage such as 4 or 5, therequired power input is smaller as the temperature difference issmaller. Because the duty to split it up between two or more trays orzones, the heat exchangers 24, 35 and 44 may be of a smaller and lessexpensive type, such as stab-in heat exchangers. If one or more stab-inheat exchangers are used, the associated conduits 22, 25, 28, 29, 33,36, 38, 39, 42, 45, 48, 51 can be eliminated or considered forschematically illustrating material flow paths in the FIGURE.

The height of the column 11 will be determined by the maximum number oftrays between any adjacent compressor suctions 19, 20 or betweenadjacent intermediate reboilers 24, 35, 44. The FIGURE shows a columnwith a single fold with a single pump 20 drawing fluid from a bottomrectification zone 3 to a top stripping zone 4. Multiple folds usingmultiple pumps like the one shown at 20 for drawing fluid from a lowerrectification zone to a top stripping zone are anticipated. Thus, theoverall height of the column can be substantially decreased using thedisclosed folded design.

The vapor from the intermediate rectification section 2 that is notcompressed by the pump 34 flows into the bottom of the top rectificationsection 1, and the liquid from the bottom of the intermediate strippingsection 5 that is not reboiled in exchanger 35 flows by gravity to thetop of the bottoms stripping section 6. As a result, the column heightmay be reduced and the total tray area can be reduced, approaching theminimum tray area from the theoretical minimum vapor flow at each tray.Further, because the overall height of the column 14 substantiallyreduced, installation costs are also reduced.

While only a certain embodiment has been set forth, alternatives andmodifications will be apparent from the above description to thoseskilled in the art. These and other alternatives are consideredequivalents and within the spirit and scope of this disclosure and theappended claims.

1. A distillation apparatus comprising: a vessel comprising a top, abottom and a continuous sidewall extending therebetween; the vesselbeing connected to a feed inlet; the vessel comprising a plurality ofseparation zones comprising a top rectification zone, a lowerrectification zone, a top stripping zone and a bottoms stripping zone;the top rectification zone being connected to a top overhead loop thatconnects the top rectification zone to a top heat pump and a bottomsheat exchanger, the bottoms stripping zone being connected to thebottoms heat exchanger where bottoms material is heated by materialpassing through the top overhead loop; and the lower rectification zonebeing connected to a lower overhead loop that connects the lowerrectification zone to a lower heat pump and a top heat exchanger, thetop stripping zone being connected to the top heat exchanger wherematerial from the top stripping zone is heated by material passingthrough the lower overhead loop.
 2. The distillation apparatus of claim1 wherein the top overhead loop is connected to a top rectification zoneproduct outlet.
 3. The distillation apparatus of claim 1 wherein thelower overhead loop is connected to a lower rectification zone productoutlet.
 4. The distillation apparatus of claim 1 wherein the bottomsstripping zone is connected to a bottoms stripping zone product outlet.5. The distillation apparatus of claim 1 wherein the top stripping zoneis connected to a top stripping zone product outlet.
 6. The distillationapparatus of claim 1 wherein the top heat pump and lower heat pump areseparate stages of a multiple stage heat pump.
 7. The distillationapparatus of claim 1 wherein each separation zone comprises a pluralityof theoretical stages.
 8. The distillation apparatus of claim 1 whereinthe feed is connected to the lower rectification zone.
 9. Thedistillation apparatus of claim 1 wherein the feed is connected to thetop stripping zone.